The present invention relates to a control for an automotive vehicle having a drivetrain with an engine and a continuously variable transmission (CVT), and more particularly to a CVT ratio control for providing a good ride feel when the vehicle moves off from a standstill position on a road with low friction coefficient.
According to a CVT ratio control for an automotive vehicle having an engine, revolution speed of an output shaft of the CVT, throttle opening degree of a throttle of the engine, and the engine speed are used as parameters to retrieve a preset shift pattern map to determine an appropriate CVT ratio. According to the preset shift pattern map, the vehicle moves off from a standstill position with the maximunm or largest CVT ratio. The CVT ratio is kept at the largest CVT ratio until the vehicle is accelerated to a predetermined vehicle speed value. At or immediately after the moment when the vehicle speed has reached the predetermined vehicle speed value, the CVT ratio changes in an upshift direction toward a minimum or smallest CVT ratio. This predetermined vehicle speed value is variable with varying throttle opening degree. The more the throttle opening degree increases, the more this predetermined vehicle speed value increases.
This control strategy does not meet with an operator's demand for ratio change if road wheels driven by the output shaft slip when the automotive vehicle is to move off from a standstill position on a road with a low friction coefficient. Under this condition, the revolution speed of the output shaft increases and deviates greatly from the actual vehicle speed at which the vehicle moves, causing the CVT ratio to change in upshift direction in response to an increase in the revolution speed of the output shaft.
If the operator releases a gas pedal linked to the engine throttle to suppress the slip of the road wheels, the revolution speed of the output shaft decreases and the CVT ratio changes in downshift direction. Thus, substantial shocks occur during the CVT ratio change in downshift direction, providing unplesant ride feel.
An object of the present invention is to improve the CVT ratio control of the above kind such that the automotive vehicle moves off from a standstill position on a road with a low friction coefficient without any substantial shift shocks inherent with the undesired downshift operation and without the above-mentioned unplesant ride feel.